A monthly access report for your site is available at http://www.your-domain-name.com/stats/ .
It is updated daily.

Raw access and error logs are available via FTP in your logs/ directory.

What is the difference between "hits", "pages", and "visits?"

A "hit" is a single HTTP request made to your site.
A "page" is a hit to a file type identified as a web page (.html, .php, .pl, etc.), as opposed to an image.
For instance, if your index page is named index.html and contains 6 image tags, loading that page with a browser would count as 7 hits but only one page.
A "visit" is a derived quantity comprised of any number of pages from a single host in a time period, 30 minutes by default. Visits may best represent the absolute number of users accessing your site in a given month.

Which statistic measures the total number of unique visitors to my site?

The closest figure is likely to be "Total Unique Sites".This is an estimate only. It is impossible to determine the exact number of people visiting your site from HTTP logs. Many (e.g. AOL) are behind proxy servers that count as one site each. On the other hand, a single dialup user with a dynamically-assigned IP address may count as thirty or more unique sites over one month.

What do the various response codes (200, 302, 304, etc.) mean?

These are HTTP protocol response codes. The standard definition can be found
here.A brief introduction can be found here.